Process for producing cumene

ABSTRACT

This cumene process involves contacting, in an alkylation zone, a first benzene recycle stream and a propylene feed stream with an alkylation catalyst to form cumene. In a transalkylation zone, a polyisopropyl benzene recycle stream and a second benzene recycle stream are contacted with a transalkylation catalyst to form additional cumene. The effluents are passed into a dividing wall distillation column. A cumene stream is removed from an intermediate point of the dividing wall fractionation column; a first benzene recycle stream is removed from a first end and a heavy aromatics stream is removed from a second end. A second benzene recycle stream is removed from an intermediate point located between the first end and the cumene stream. A polyisopropyl benzene stream is removed from an intermediate point of located between the second end and the cumene stream.

FIELD OF THE INVENTION

The invention is directed to a process for producing cumene where theproduct is separated from the reaction mixture using a dividing walldistillation column.

BACKGROUND OF THE INVENTION

Cumene, or isopropylbenzene, is a valuable product that is used mainlyfor the manufacture of phenol and acetone. Cumene has been produced by acatalytic process using a solid phosphoric acid that is made byimpregnating kieselguhr with phosphoric acid. Now, zeolitic catalystsare used to produce higher quality cumene at a lower investment cost.

In a typical commercial process for the production of cumene, liquidbenzene and liquid propylene are charged into an alkylation zonecontaining one or more reactors containing an alkylation catalyst. Inorder to minimize the production of dialkylated products of benzene ithas been the practice to maintain a molar excess of benzene throughoutthe reaction zone ranging from about 4:1 to about 16:1, and morepreferably about 8:1 of benzene to propylene. Two competing reactionswith the desired production of isopropylbenzene have created problems insome commercial processes. One of these has been the formation ofdialkylated benzenes such as di- and triisopropylbenzene rather than thedesired monoalkylated product. This competing reaction may be partiallycontrolled by employing large molar excesses of benzene. However, atransalkylation reactor is employed to react dialkylated benzenes withbenzene to form additional cumene. The other competing reaction causinglosses in the yield of cumene based on propylene reactant charged is theformation of oligomers of propylene such as propylene dimer and trimerwhich occur to a limited extent even with the large molar excesses ofbenzene present. Propylene trimers and some of the propylene tetramersboil with cumene. Since the presence of these olefins interfere with theoxidation reaction used to make phenol from cumene, this oligomerizationside reaction must be minimized to make a high purity product.

The alkylator and transalkylator effluents undergo separation operationsto separate benzene, cumene product, polyisopropylbenzene, andby-product streams using distillation columns. Traditionally threedistillation columns are used. The first is typically a benzene column,used to recover excess benzene from the reactor effluents. The benzenecolumn overhead, which is largely benzene, is typically recycled to thealkylator and transalkylator. The second distillation column istypically a cumene column used to recover the cumene product from thebenzene column bottoms. The cumene product is typically the net overheadfrom the cumene column. The cumene product may be used in applicationssuch as phenol or acetone processes, or may be sent to storage. Thethird distillation column is usually a polyisopropylbenzene column usedto recover polyisopropylbenzene recycle stream from the cumene columnbottoms. Polyisopropylbenzene is recovered as overhead from thepolyisopropylbenzene column and is typically recycled to thetransalkylator. The high boiling bottoms, the heavy ends, is usuallycooled and sent to storage.

Current process flow schemes are improved by replacing the benzenecolumn and the cumene column with a single dividing wall column. Theresulting advantages include a significant savings in the energyrequirement and the total number of stages, a higher cumene purity and areduction in benzene loss. Additional advantages include a reduction incapital costs associated with the reduced number of stages, reduction inexchanger surface area, and a reduction in the equipment count.

The dividing wall or Petyluk configuration for fractionation columns wasinitially introduced some 50 years ago by Petyluk et al. A recentcommercialization of a fractionation column employing this techniqueprompted more recent investigations as described in the articleappearing at page s14 of a Supplement to The Chemical Engineer, 27 Aug.1992.

The use of dividing wall columns in the separation of hydrocarbons isalso described in the patent literature. For instance, U.S. Pat. No.2,471,134 issued to R. O. Wright describes the use of a dividing wallcolumn in the separation of light hydrocarbons ranging from methane tobutane. U.S. Pat. No. 4,230,533 issued to V. A. Giroux describes acontrol system for a dividing wall column and illustrates the use of theclaimed invention in the separation of aromatics comprising benzene,toluene and orthoxylene.

Using a dividing wall column in the a cumene production process providessignificant advantages over cumene production processes that do notemploy a dividing wall fractionation column, as shown below.

SUMMARY OF THE INVENTION

A cumene generation process having a dividing wall fractionation zonehas been developed. The process involves contacting, in an alkylationzone, a feed stream comprising at least propylene and a first benzenerecycle stream comprising at least benzene with an alkylation catalystunder alkylation conditions to convert at least a portion of thepropylene and benzene into cumene and form an alkylation zone effluentcomprising benzene and cumene. Also, in a transalkylation zone, apolyisopropyl benzene recycle stream comprising at least polyisopropylbenzene and a second benzene recycle stream comprising at least benzeneare contacted with a transalkylation catalyst under transalkylationconditions to convert at least a portion of the polyisopropyl benzeneand benzene into cumene and form a transalkylation zone effluentcomprising benzene and cumene. The alkylation zone effluent and thetransalkylation zone effluent are passed into a dividing wallfractionation column which is operated at fractionation conditions. Thedividing wall fractionation column is divided into at least a first anda second parallel fractionation zone by a dividing wall, with the firstand the second fractionation zones each having an upper and a lower endlocated within the fractionation column, with the first and secondfractionation zones being in open communication at their upper ends withan undivided upper section of the fractionation column and in opencommunication at their lower ends with an undivided lower section of thefractionation column. The effluents from the two reactions zones enterthe dividing wall column at one or more intermediate points of the firstfractionation zone.

A stream comprising cumene, or isopropyl benzene, is removed from anintermediate point of the second fractionation zone of the dividing wallfractionation column. A first benzene recycle stream is removed from afirst end of the dividing wall fractionation column, and a heavyaromatics stream is removed from a second end of the dividing wallfractionation column. The second benzene recycle stream is removed froman intermediate point of the second fractionation zone of the dividingwall fractionation column located between the first end of the columnand intermediate point where the cumene stream is removed. Apolyisopropyl benzene stream is removed from an intermediate point ofthe second fractionation zone of the dividing wall fractionation columnlocated between the second end of the column and the intermediate pointwhere the cumene stream is removed. Optionally a semi-dry benzene streammay be removed from an intermediate point of the second fractionationzone of the dividing wall fractionation column located between the firstend of the column and the location where the second benzene recyclestream is removed from the dividing wall column.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is a schematic illustration of one embodiment of the presentinvention. The FIGURE does not show a number of details for the processarrangement such as pumps, compressors, valves, stabilizers and recyclelines which are well known to those skilled in the art.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the FIGURE, the process of the invention requires tworeactors, an alkylation reactor 2, or alkylator, and a transalkylationreactor 4, or transalkylator. Propylene and benzene feedstocks 6 and anexcess of benzene 8 are introduced to alkylator 2. A typical propylenefeedstock may be an almost pure polymer grade material or can containsignificant amounts of propane, as typically found in refinery-gradepropylene. A typical benzene feedstock may contain benzene (99.9 wt.-%mm.) and toluene (0.05 wt.-% min). Alkylation reactors may be operatedin the vapor phase, liquid-phase or mixed-phase. It is preferred tooperate the alkylation reactor in the liquid phase. At the lowertemperatures of the liquid phase operation, xylene impurities are notproduced and a cumene product of superior quality is produced. Suitabletemperatures include from about 100 to about 310° C. and from about 150to about 280 ° C. Suitable pressures include from about 8 to 50 bar(gage) and from about 10 to 38 bar (gage). Alkylation reactor 2 containsan effective amount of alkylation catalyst. Suitably alkylationcatalysts include solid acid catalysts and preferably a solid oxidezeolite. Examples are zeolite beta, zeolite X, zeolite Y, mordenite,faujasite, zeolite omega, UZM-8, MCM-22, MCM-36, MCM-49 and MCM-56. Inone embodiment, the temperature of the alkylation reactor is selectedfrom the range of 100° C. to 310° C. (212 to 590° F.) and the pressureis selected from the range of 800 to 5000 kPa (gauge) (116 to 725 psig).In a more specific embodiment the temperature is in the range of 120 to280° C. (248 to 536° F.) and the pressure is in the range of from about1000 to 3800 kPa (gauge) (145 to 551 psig). Alkylation reactors,operating conditions and catalysts are known in the art and not furtherdiscussed here.

In alkylation reactor 2, the benzene is alkylated with the propylene toform isopropylbenzene or cumene. Some polyisopropyl benzenes, which aremainly di- and tri-substituted propylbenzenes, are also formed. Benzeneis fed to the alkylator in excess so that virtually all the propylene isreacted. The alkylation reactor effluent 10 contains primarily benzene,cumene and polyisopropyl benzenes.

Transalkylation reactor 4 is used to transalkylate the polyisopropylbenzene produced in the alkylation reactor and recycled in line 42 withbenzene recycled in line 20 to form additional cumene. Suitableconditions and catalysts may be the same as described for the alkylationreactor. In one embodiment, the temperature is selected from the rangeof 100° C. to 270° C. (212 to 518° F.) and the pressure is selected fromthe range of 800 to 5100 kPa (116 to 740 psia). In another more specificembodiment, the temperature is about 129° C. (264° F.) and the pressureranges from about 1000 to 3900 kPa (145 to 565 psig). Transalkylationreactors, operating conditions and catalysts are known in the art andnot further discussed here. The transalkylation effluent 12 fromtransalkylation reactor 4 contains primarily benzene, cumene, andpolyisopropyl benzene.

Both alkylation effluent 10 and transalkylation effluent 12 areintroduced to a dividing wall distillation column 14. Within thedividing wall distillation column are two parallel fractionation zones.A first fractionation zone occupies a large portion of the left-handside of the mid-section of the distillation column. Note that the terms“left-hand” and “right-hand” are used herein as relative to thedrawings. In actual practice the placement of the zones as to the leftside or the right side of the column is not critical. This firstfractionation zone is separated from a parallel second fractionationzone occupying the other half of the column cross section by asubstantially fluid tight vertical wall 16. The vertical wall is notnecessarily centered in the column and the two fractionation zones maydiffer in cross sectional area or shape. The vertical wall divides alarge vertical portion of the column into two parallel fractionationzones. The two zones are isolated from each other for the height of thiswall, but communicate at both the top and bottom ends of the column.There is no direct vapor or liquid flow between the two fractionationzones through the dividing wall, but the upper end of the fractionationzone is open to the internal volume of the distillation columncontaining an undivided fractionation zone preferably having additionaltrays. Liquid may pass under the dividing wall at the bottom of the twofractionation sections although vapor flow is preferably restricted orcontrolled. Thus, vapor and liquid can freely move around the wallbetween the two portions of the column.

During operation, the components of both effluents are separated in thefirst fractionation zone with the more volatile compounds moving upwardout of the left-hand first fractionation zone and emerging into theundivided upper portion of the distillation column. As with the firstfractionation zone, the upper end of the right-hand second zone is inopen communication with the upper section of the distillation columnwhich may optionally contain additional fractionation trays extendingacross the entire column cross section.

The components of the effluents will separate according to boiling pointor relative volatilities, which is the main factor in determining theirbehavior in the distillation column. The component having a relativelylow boiling point is the benzene from each of the effluents. Themid-range boiling component is the desired product, cumene. Thecomponents having relatively higher boiling points are the polyisopropylbenzenes, and the components having the highest boiling points are anyheavy aromatics.

The transalkylation effluent 12 and the alkylation effluent 10 areintroduced into a first vertical fractionation zone occupying a largeportion of the left-hand side of the midsection of the distillationcolumn. The effluents may be combined before being introduced, butadvantages may be realized by introducing the effluents at differentheights along the dividing wall distillation column. For example, thealkylation reactor effluent may contain a higher concentration ofpolyisopropyl benzene and therefore it may be advantageous to introducethe alkylation reactor effluent at a relatively lower height along thecolumn as compared to the transalkylation reactor effluent.

The benzene present in the effluents is driven upwards in the firstfractionation zone and enters the top section of the column where it isremoved in overhead 18 and sidedraw 20. Benzene in overhead 18 isremoved from the top of the dividing wall column and passed through anoverhead condenser 38 to form liquid delivered to receiver 22. Receiver22 may also have vent stream 21. A liquid phase stream of benzene 26 isremoved from the receiver and divided into a first portion 28 which isreturned to the top of the dividing wall fractionation column as refluxand a second portion 8 which is recycled to the alkylation reactor 2 asthe first benzene recycle stream. Benzene may also be removed fromdividing wall column in stream 20 which is a sidedraw. Benzene stream 20may be recycled to transalkylation reactor 4 as the second benzenerecycle stream. As used herein the term “rich” is intended to indicate aconcentration of the indicated compound or class of compounds greaterthan 50 and preferably greater than 75 mol percent.

The bottom of the dividing wall fractionation column 14 also comprisesan undivided fractionation zone. This zone can receive liquid drainingfrom both the first and second fractionation zones. This liquid issubjected to distillation which drives the cumene and polyisopropylbenzene upwards as vapor while concentrating the less volatile tars intoa bottoms liquid 34 that is removed from dividing wall fractionationcolumn 14. This separation is effected through the use of a reboiler 36providing vapor to the bottom undivided fractionation zone. The productcumene stream 32 is withdrawn from the dividing wall fractionationcolumn in a side draw from the right-hand side fractionation zone fromthe right-hand side fractionation zone. The byproduct polyisopropylbenzene stream 42 is withdrawn from the dividing wall fractionationcolumn in a side draw. Polyisopropyl benzene stream 42 is withdrawn at alower height of the column as compared to the cumene stream 32. Abottoms stream containing heavy aromatics is removed from the dividingwall distillation column and a portion of which is passed troughreboiler 36. The remainder is the net heavy aromatics stream 34.

Optionally, a semi dry benzene stream may be removed from the dividingwall fractionation column 14 in stream 25 as a side draw. Stream 25 islocated at a height of dividing wall fractionation column between thatof the first and second benzene recycle streams 8 and 20. Two benzenestreams are withdrawn from the dividing wall fractionation columnbecause the catalysts typically used in the alkylation reactor cantolerate some water present during the alkylation reaction, but thecatalysts typically used in the transalkylation reactor are lesstolerant of water. Therefore, the overhead contains benzene which may besaturated with water and is appropriate for the alkylation reactor,while the side draw contains semi-dry benzene which is appropriate forthe transalkylation reactor.

The dividing wall fractionation column may be further equipped withstabbed-in reboilers and heat recovery exchangers located at strategicpositions along the column. Examples of suitable locations for stabbedin reboilers include between the polyisopropyl benzene stream and thesecond end, and between the second benzene stream and the first end ofthe dividing wall distillation column. Such reboilers may be utilized tooptimize the column diameter and energy consumption.

In a more specific embodiment of the invention, the undivided bottomsection of the dividing wall fractionation column is depicted asseparated from the two parallel fractionation zones by a gas flowcontrol or gas trap out tray located just below the bottom of the wall.A slight gap at this point allows horizontal liquid flow between theparallel fractionation zones. This tray may have liquid sealedperforations allowing the normal downward flow of liquid, but itsstructure is such that the upward flow of vapor is at least greatlyrestricted or controlled. The tray may totally block the upward vaporflow. The use of this tray may provide a means to positively control thepartition of the upward gas flow between the two fractionation zones,which is a prime means of controlling performance of the two zones. Thetotal vapor from the column bottom, therefore, preferably removed fromthe column via a line and split between two lines which feed the vaporto the bottom of the two parallel fractionation zones separately. Thegas flow may be controlled by one or more flow control valves or byadjusting the relative liquid levels in the bottom of the two zones.This is described in some detail in U.S. Pat. No. 4,230,533.

It is believed that the separation performed in the present inventionwould compare favorably to that achieved using a conventional schemeemploying a benzene column followed by a cumene column and apolyisopropyl benzene column. An investigation would most likelyindicate that the three conventional fractionation columns (the benzenecolumn, the cumene column and the polyisopropyl benzene column) could bereplaced with the dividing wall column of the present invention toprovide significant benefits and costs savings. It is expected that thereboiling duty may decrease as well as the total number of requiredstages. Additionally, one might expect there to be a small increase incumene recovery which translates into a smaller loss of cumene product.The total capital cost for the dividing wall column and its associatedequipment, including heat exchangers, pumps, and receiving vessels, isexpected to be less than the total for the columns it replaces,including the equipment associated with these columns. Several factorscontribute to this cost reduction. First, as mentioned above, it isexpected that the total reboiling duty may decrease as well as the totalnumber of stages. The reduction in reboiling duty leads to a reductionin the size of the heat exchangers required for condensing and reboilingthe column. In addition, the total pieces of equipment associated with adividing fractionation wall column (including pumps, heat exchangers,and vessels) are fewer than the total required for the separatefractionation columns that it replaces. Fewer pieces of equipment for amore efficient separation will lead to a lower capital cost. Thereduction in equipment count also leads to other less tangible benefits.The total plot space requirement for the separation is reduced. Inaddition, the total inventory of hydrocarbon is reduced, therebyproviding, relatively, a more safe process unit.

1. A cumene generation process using a dividing wall fractionation zone,said process comprising: contacting, in an alkylation zone, at leastpropylene and benzene and a first benzene recycle stream comprising atleast benzene with an alkylation catalyst under alkylation conditions toconvert at least a portion of the propylene and benzene into cumene andform an alkylation zone effluent comprising benzene and cumene;contacting, in a transalkylation zone, a polyisopropyl benzene recyclestream comprising at least polyisopropyl benzene and a second benzenerecycle stream comprising at least benzene with a transalkylationcatalyst under transalkylation conditions to convert at least a portionof the polyisopropyl benzene and benzene into cumene and form atransalkylation zone effluent comprising benzene and cumene; passing thealkylation zone effluent and the transalkylation zone effluent into adividing wall fractionation column operated at fractionation conditionsand divided into at least a first and a second parallel fractionationzone by a dividing wall, with the first and the second fractionationzones each having an upper and a lower end located within thefractionation column, with the first and second fractionation zonesbeing in open communication at their upper ends with an undivided uppersection of the fractionation column and in open communication at theirlower ends with an undivided lower section of the fractionation column,and with the alkylation zone effluent and the transalkylation zoneeffluent entering the column at intermediate points of the firstfractionation zone, wherein the transalkylation zone effluent isintroduced into the dividing wall fractionation column at anintermediate height of the dividing wall fractionation column betweenthe introduction point of the alkylation zone effluent and a first endof the dividing wall fractionation column; removing a stream comprisingcumene from an intermediate point of the second fractionation zone ofthe dividing wall fractionation column; removing the first benzenerecycle stream from the first end of the dividing wall fractionationcolumn; removing a heavy aromatic-rich stream from a second end of thedividing wall fractionation column; removing the second benzene recyclestream from an intermediate point of the second fractionation zone ofthe dividing wall fractionation column located between the first end ofthe dividing wall fractionation column and the cumene stream; andremoving a polyisopropyl benzene-rich stream from an intermediate pointof the second fractionation zone of the dividing wall fractionationcolumn located between the second end of the dividing wall fractionationcolumn and the cumene stream.
 2. The process of claim 1 wherein thealkylation zone is operated at a pressure in the range of 800 to 5100kPa (116 to 740 psia) and a temperature in the range of 120 to 280° C.(248 to 536° F.).
 3. The process of claim 1 wherein the transalkylationzone is operated at a pressure in the range of 800 to 5100 kPa (116 to740 psia) and a temperature in the range of 100 to 200° C. (212 to 392°F.).
 4. The process of claim 1 wherein the stream comprising cumene fromthe dividing wall fractionation column comprises 99.955 wt. % cumene. 5.The process of claim 1 wherein the stream comprising cumene from thedividing wall fractionation column comprises no more than about 4 wt.ppm benzene.
 6. The process of claim 1 further comprising passing thestream comprising cumene to a phenol generation process.
 7. The processof claim 1 further comprising passing the stream comprising cumene to anacetone generation process.